Matt, thermoformable, IR-reflective polyester film

ABSTRACT

Biaxially oriented polyester films formed from a crystallizable polyester having increased diethylene glycol content and/or increased polyethylene glycol content, and/or increased isophthalic acid content, preferably polyethylene terephthalate. The films of the invention further include at least one IR-reflective pigment and one UV stabilizer. The inventive films feature adjustable mattness, diffuse scattering power for visible light, high light transmittance and IR reflectance and good thermoformability, and are suitable for thermally protective coatings or thermally protective packaging.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims prriority to German parent application 10 2004032 595.2 which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a matt, IR-reflective biaxially orientedpolyester film whose thickness is in the range from 3 to 200 μm. Thefilm comprises at least one lamellar, IR-reflective pigment and one UVstabilizer, and features good orientability, adjustable mattness,diffuse scattering power for visible light, high light transmittance,very good longitudinal and transverse mechanical properties, goodthermoformability, very little yellowing after a prolonged period ofinsolation, and cost-effective production. The invention further relatesto a process for the production of this film and to its use.

BACKGROUND OF THE INVENTION

Biaxially oriented polyester films in the range of thickness from 3 to200 μm are well known in the form of transparent, matt, and white films.These films are produced with pigments, e.g. silicon dioxide, calciumcarbonate, barium sulfate, kaolin, titanium dioxide, aluminum oxide, orcombinations thereof. These films are generally not thermoformable, donot exhibit diffuse scattering power for visible light, and certainly donot reflect IR radiation.

EP 0 659 198 B1 describes composite materials intended for solarradiation purposes, screening purposes, and, respectively, filteringpurposes, uses described being agriculture and horticulture. These filmsare comprised of a transparent polymer selected from the group of low-or high-density polyethylene, ethylene-vinyl acetate copolymer,polytetrafluoroethylene, polyvinylidene dichloride, polyvinyl chloride,polycarbonate, polymethyl methacrylate, and mixtures thereof. Thesefilms comprise from 0.1 to 30% by weight, based on the polymer of aninterference pigment which is comprised of a lamellar material, whichhas been coated with one or more metal oxides. This film featuresselective screening of radiation, which has a favorable effect on plantgrowth.

EP 0 791 620 A2 describes films for agricultural applications comprisedof polyetheramide block copolymers, of thermoplastic polyesterelastomers, or of thermoplastic polyurethanes, having water-vaporpermeability of from 100 to 25 000 g/m²/24 h, transmittance of from 20to 90% for visible light, and transmittance of from 5 to 90% for thermalradiation with wavelength of 5 μm. The films comprise lamellar inorganicparticles which have been coated with a substance with a high refractiveindex.

The films described have not been biaxially oriented and therefore haveinadequate longitudinal and transverse mechanical properties, makingthem unusable for many applications where the demands are high ultimatetensile strength, high modulus of elasticity, high tensile strain atbreak values, and high tensile stress values to generate 5% tensilestrain (F₅ value).

The films described are moreover unsuitable for the thermoformingprocess, because the result would be uncontrolled orientation of thepigments partially orientated via melt flow. This would have anuncontrolled effect on optical properties of the film, e.g. gloss, haze,and mattness, via vacuole formation involving pigments and orientationof the film.

These films moreover yellow as a result of prolonged insolation, makingthem unsightly. The mechanical properties of the films are also impairedby insolation, and the films therefore become brittle after a very shorttime. This effect, brought about by the short-wave fraction of sunlight,is very much more pronounced in pigmented films, in particular in filmswhich comprise photoactive metal oxides, e.g. titanium dioxide, than inunpigmented films.

EP 1 251 369 A2 describes biaxially oriented multilayer polyester filmswhose base layer comprises an IR-absorbent dye, and which are suitablefor use as IR filters. This dye has an absorption maximum at from 800 to1000 nm, and gives the film maximum transmittance of 30% at 950 nm.

The use of standard polyesters combined with biaxial orientation of thefilms described makes them completely unsuitable for thermoformingprocesses.

The films described are moreover unsuitable for outdoor applicationsbecause prolonged insolation impairs their mechanical and opticalproperties. Because the IR-absorbing dyes used are sensitive organicmolecules which are decomprised by insolation, the IR-filter action ofthe films described rapidly reduces in outdoor applications.

SUMMARY OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

It was therefore an object of the present invention to provide a matt,IR-reflective, thermoformable, polymer film which does not have thedisadvantages known from the prior art for various types of film.

A further intention is that this film should not only be cost-effectiveto produce and have good longitudinal and transverse orientability, butshould also have adjustable mattness, good thermoformability, and,primarily, very good mechanical properties, i.e. high longitudinal andtransverse ultimate tensile strength, high longitudinal and transversemodulus of elasticity, high longitudinal and transverse tensile strainat break, and high longitudinal and transverse tensile stress togenerate tensile strain of 5% (F₅ value), and in particular should havehigh light transmittance, diffuse scattering power for visible light,very low permeability to IR radiation, and very little change inyellowness index after prolonged insolation.

DETAILED DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

This object is achieved via a biaxially oriented polyester film whichcomprises as main constituent crystallizable polyester whose DEG contentis preferably greater than or equal to 1% by weight and/or whose IPAcontent is preferably greater than or equal to 2% by weight, and/orwhose PEG content is preferably greater than or equal to 1% by weight.This film is characterized by comprising at least one IR-reflective,preferably lamellar, pigment, and by comprising at least one UVstabilizer. Pigment and UV stabilizer are preferably added by means ofmasterbatch technology during film production. The thickness of theinventive film is preferably from 3 to 200 μm, in particular from 10 to150 μm. DEG is diethylene glycol, PEG is polyethylene glycol, and IPA isisophthalic acid. The film may comprise one or more crystallizablepolyesters as a main constituent.

The following applies to the preferred properties of the film:

Good orientability includes the capability of the film to give excellentlongitudinal and transverse orientation during its production, withoutbreak-offs.

Cost-effective production includes the capability of the raw materialsor raw material components needed for producing the film to be driedusing commercial industrial dryers of the prior art. It is importantthat the raw materials here do not cake and do not undergo thermaldegradation. Among these prior-art industrial dryers are vacuum dryers,fluidized-bed dryers, and fixed-bed dryers (tower dryers).

Thermoformability means that the film can be thermoformed oncommercially available thermoforming machines without uneconomicpredrying to give complex and large-surface-area moldings.

Adjustable mattness means that the gloss of the film, and also itsroughness, can be adjusted during the film production process either byway of the dimensions of the pigment (longitudinal dimension:transversedimension) or else by way of the processing and orientation parameters.Mattness also means that the inventive film has vacuoles whose length isin the range from 0.5 to 20 μm.

Among the good mechanical properties are a high modulus of elasticity(greater than 3200 N/mm² longitudinally=in machine direction (MD),greater than 3500 N/mm² transversely (TD)), high ultimate tensilestrength values (more than 100 N/mm² in MD; more than 130 N/mm² in TD),high tensile strain at break values (more than 100% in MD and TD), andhigh tensile stress values to generate 5% tensile strain (F₅ value; morethan 100 N/mm² in MD and TD).

High light transmittance means that the light transmittance is greaterthan (>) 60%.

Diffuse scattering behavior means that image sharpness is smaller than(<) 85%.

Very low permeability to IR radiation means that transmittance forelectromagnetic radiation in the wavelength range from 750 to 1300 nm issmaller than 50%.

Very little yellowing means that the change in yellowness index (ΔYID)of the film after 5000 h of weathering in a Weather-Ometer is smallerthan (<) 6.

These abovementioned values are preferred parameters for the inventivefilm.

The film comprises, as main constituent, a crystallizable polyester.According to the invention, a crystallizable polyester is crystallizablehomopolyesters, crystallizable copolyesters, crystallizable blends ofvarious polyesters, crystallizable recycled material, and othervariations on crystallizable polyesters.

Examples of suitable crystallizable or semicrystalline polyesters arepolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT),preference being given to polyethylene terephthalate (PET).

It is significant for the invention that DEG content and/or PEG contentand/or IPA content of the crystallizable polyester is higher than thatof standard polyesters.

The crystallizable polyester preferably has a DEG content of from 1.0 to10% by weight, preferably from 1.2 to 5.0% by weight, and in particularfrom 1.3 to 3.0% by weight, and/or a PEG content of from 1.0 to 10% byweight, preferably from 1.2 to 5.0% by weight, in particular from 1.3 to3.0% by weight, and/or an IPA content of from 2.0 to 20% by weight,preferably from 3.0 to 15% by weight, and in particular from 5.0 to 10%by weight.

It was more than surprising that a higher DEG content and/or PEG contentand/or IPA content in comparison with standard polyester makes the filmscapable of cost-effective thermoforming on commercially availablethermoforming plants, giving excellent reproduction of detail, withoutany measurable alteration of optical properties, such as gloss andmattness.

The standard viscosity SV (DCA) of the polyester is preferably from 600to 1000, preferably from 700 to 900.

Preferred starting materials for production of the inventive film arecrystallizable polyesters whose crystallite melting point Tm is from 180to 365° C. or above, preferably from 180 to 310° C., whosecrystallization temperature range T_(c) is from 75 to 280° C., whoseglass transition temperature T_(g) is from 65 to 130° C. (determined viadifferential scanning calorimetry (DSC) at density of from 1.10 to 1.45g/cm³ (determined via DIN 53479).

Bulk density (measured to DIN 53466) is generally from 0.75 to 1.0kg/dm³, preferably from 0.8 to 0.90 kg/dm³.

The polydispersity (=Mw:Mn ratio) of the polyester, measured via gelpermeation chromatography (GPC) is preferably from 1.5 to 4.0,particularly preferably from 2.0 to 3.5.

“Main constituent” means that the proportion of the crystallizable orsemicrystallizable polyester(s) is preferably from 50 to 99% by weight,particularly preferably from 75 to 95% by weight, based in each case onthe total weight of the film. The remaining content may be made up notonly by the lamellar, IR-reflective pigment but also by other pigmentsor additives conventional for biaxially oriented polyester films.

The inventive film is preferably a single-layer film, and may have beencoated with various copolyesters or adhesion promoters.

The thickness of the inventive film may vary within wide limits, anddepends on the intended application. It is preferably from 3 to 200 μm,in particular from 5 to 150 μm, and particularly preferably from 10 to100 μm.

The inventive film can moreover be recycled without polluting theenvironment, and the film produced here from the recycled materialexhibits practically no impairment of optical properties (in particularyellowness index) or of mechanical properties when compared with a filmproduced from virgin starting materials.

The inventive film comprises at least one, preferably lamellar,IR-reflective pigment, and at least one UV stabilizer. It isadvantageous for the pigment to be fed by way of masterbatch technologydirectly during film production, the concentration of the pigment herebeing from 0.5 to 50% by weight, with preference from 1.0 to 25% byweight, and in particular from 1.5 to 15% by weight, based on the weightof the film.

Examples of suitable lamellar, IR-reflective pigments arephyllosilicates, e.g. kaolin, talc, or feldspar which have been coatedwith metal oxides, e.g. titanium dioxide, zirconium dioxide, aluminumoxide, and/or silicon dioxide. The size of the pigments is preferablyfrom 1 to 100 μm. Pigments of this type are described in EP 0414049 ofMerck and Hyplast, particular preference being given here to IRIODIN®SHR 870 and IRIODIN® SHR 875 from Merck, Germany.

In principle, all, preferably lamellar, IR-reflective pigments aresuitable for the inventive purpose.

It is important that these pigments are orientated via the orientationof the film and, as a function of orientation, form vacuoles of varyingsize. These vacuoles determine the appearance of the film and thediffuse scattering power for visible light. This means that, givenidentical pigment content in the film, its optical properties and thediffuse scattering power can be adjusted by way of the stretchingtemperatures and the stretching parameters during film production.

It was more than surprising that the production and the formation ofvacuoles is not impaired by the high DEG content and/or high PEG contentand/or high IPA content of the polyester.

Since these vacuoles are optimized via the orientation process and havebeen stabilized via the heat-setting process, no adverse effect onoptical properties occurs during the thermoforming process.

Electromagnetic radiation in the wavelength range from 750 to 1300 nm(IR) is in particular responsible for increases in indoor temperatures.This radiation likewise impairs heat-sensitive goods.

The film of the invention reflects radiation in this wavelength range,so that the transmittance of the film is preferably smaller than 50% atfrom 750 to 1300 nm. A consequence is less rise in indoor temperaturesand the possibility of packaging heat-sensitive goods, if the inventivefilm is used appropriately.

The inventive film comprises not only the IR-reflective pigment but atleast one UV stabilizer.

Light, in particular the ultraviolet content of insolation, i.e., thewavelength region from 280 to 400 nm, induces degradation inthermoplastics, as a result of which their appearance changes due tocolor change or yellowing, and there is also an extremely adverse effecton mechanical/physical properties of the films comprised of thethermoplastics.

Inhibition of this photooxidative degradation is of considerableindustrial and economic importance, since otherwise there are drasticlimitations on the applications of numerous thermoplastics.

The absorption of UV light by polyethylene terephthalates, for example,starts only just below 360 nm, increases markedly below 320 nm, and isvery pronounced at below 300 nm. Maximum absorption occurs at between280 and 300 nm.

In the presence of oxygen it is mainly chain cleavage which occurs, butthere is no crosslinking. The predominant photooxidation products inquantity terms are carbon monoxide, carbon dioxide, and carboxylicacids. Besides direct photolysis of the ester groups, consideration hasto be given to oxidation reactions which likewise form carbon dioxidevia peroxide radicals.

In the photooxidation of polyethylene terephthalates there can also becleavage of hydrogen at the position a to the ester groups, givinghydroperoxides and decomposition products of these, and this may beaccompanied by the chain cleavage (H. Day, D. M. Wiles: J. Appl. Polym.Sci. 16, 1972, p. 203).

UV stabilizers, i.e. light stabilizers which are UV absorbers, arechemical compounds which can intervene in the physical and chemicalprocesses of light-induced degradation. Carbon black and other pigmentscan give some protection from light. However, these substances areunsuitable for pale-colored or indeed opaquely colored films, since theycause discoloration or color change.

UV stabilizers suitable as light stabilizers are those which preferablyabsorb at least 70%, with preference 80%, particularly preferably atleast 90%, of the UV light in the wavelength region from 180 to 380 nm,preferably from 280 to 350 nm. These are particularly suitable if theyare thermally stable in the temperature range from 260 to 300° C., thatis to say they do not decompose to give cleavage products and do notcause evolution of gases. Examples of UV stabilizers suitable as lightstabilizers are 2-hydroxybenzophenones, 2-hydroxybenzotriazoles,organonickel compounds, salicylic esters, cinnamic ester derivatives,resorcinol monobenzoates, oxanilides, hydroxybenzoic esters,benzoxazinones, and sterically hindered amines and triazines, and amongthese preference is given to the 2-hydroxybenzotriazoles, thebenzoxazinones, and the triazines.

It was surprising that the use of UV stabilizers in combination with theIR-reflective pigments described above gives useful films with excellentproperties.

The inventive film protects, by way of example, packed goods fromshort-wavelength light in the wavelength range from 200 to 380 nm, itstransmittance preferably being smaller than (<) 50% for radiation in thewavelength range from 750 to 1300 nm, the results being prevention ofundesired increase in the temperatures of the packed product andresultant premature spoiling. When used in the construction sector thefilm moreover provides thermal protection, i.e. reduces the amount ofincrease in indoor temperatures, without yellowing on exposure toprolonged insolation.

The literature discloses UV stabilizers which absorb UV radiation andthus provide protection. However, when these known commerciallyavailable UV stabilizers are used it is found that the UV stabilizer hasinadequate thermal stability and, at temperatures of from 200 to 240°C., decomposes or causes evolution of gas. It would therefore have beennecessary to incorporate large amounts (from about 10 to 15% by weight)of UV stabilizer into the film so that it gives really effectiveabsorption of UV light. However, at these high concentrations the filmdiscolors markedly even before the production process is complete.Mechanical properties, too, are adversely affected. On orientation,exceptional problems occur, e.g. break-offs due to inadequate filmstrength (low ultimate tensile strength and modulus of elasticity), diedeposits leading to profile variations, deposits of UV stabilizer andpigment on the rolls, leading to impairment of optical properties(marked haze, adhesion-related defects, inhomogeneous surface), anddeposits in the stretching and setting frame, which contaminate thefilm. It was therefore surprising that even low concentrations of thepreferred UV stabilizers achieve excellent UV protection, and that afilm with low yellowness index is obtained. Surprisingly, the presenceof the UV stabilizer has absolutely no effect on protection fromradiation in the wavelength range from 750 to 1300 nm.

In one very particularly preferred embodiment, the inventive filmcomprises, as UV stabilizer,2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol of the formula

or2,2′-methylenebis[6-benzotriazol-2-yl-4-(1,1,2,2-tetramethylpropyl)phenol]of the formula

or 2,2′-1,4-phenylenebis(3,1-benzoxazin-4-one) of the formula

These, and other inventive UV stabilizers, are commercially available.

In another embodiment, it is also possible to use a mixture of UVstabilizers, or a mixture of these UV stabilizers, or a mixture of atleast one of these UV stabilizers with other UV stabilizers. The totalconcentration of light stabilizer is preferably from 0.1 to 5.0% byweight, particularly preferably from 0.5 to 3.0% by weight, based on theweight of the film.

The inventive film may comprise not only the IR-reflective pigment andthe UV stabilizer but also other conventional additives, such as fillersand antiblocking agents. They are advantageously added to the polymer orpolymer mixture prior to the start of the melting process.

Other additives which may be selected are a mixture of two or moredifferent antiblocking agents, or a mixture of antiblocking agents ofthe same constitution but of different particle size. The conventionalproportions of the particles may be added to the film, e.g. in the formof a glycolic dispersion, during the polycondensation process or by wayof masterbatches during extrusion. Pigment concentrations of from 0.0001to 10% by weight, based on the weight of the film, have proven to beparticularly suitable.

For particular applications it can be advantageous for the surface ofthe film to be chemically pretreated with an acid. Particularly suitablecompounds for this process known as adhesion-promotion by etching aretrichloroacetic acid, dichloroacetic acid, or hydro-fluoric acid, whichact on the surface for a short period (from 5 to 120 seconds) and thenare removed by means of air knives. This gives the film a very reactive,amorphous surface.

The inventive film may have at least one further functionality. Theadditional functionality is preferably that the film has been corona- orflame-treated and/or rendered flame-retardant, and/or coated on one orboth sides.

In the case of single- or double-sided coating of the film, thethickness of the dried coating is generally from 5 to 100 nm, preferablyfrom 20 to 70 nm, in particular from 30 to 50 nm. It is preferablyapplied in-line, i.e. during the film-production process, advantageouslyprior to the transverse orientation process. Particular preference isgiven to application by means of the reverse gravure-roll coatingprocess, which can apply the coating extremely homogeneously at thelayer thickness mentioned. The coatings are applied—preferably inaqueous form—in the form of solutions, suspensions, or dispersions, inorder to give the film surface additional functionalities. Examples ofsubstances or compositions which give additional functionality areacrylates (see WO 94/13476), ethylene-vinyl alcohols, polyvinylidenechloride, water glass (sodium silicate), aminosilanes (described in EP 0359 017 and its United States equivalent U.S. Pat. No. 4,939,035 or EP 0346 768 and its United States equivalent U.S. Pat. No. 4,898,786),hydrophilic polyesters (PET/IPA polyesters as described in EP-A-144 878or U.S. Pat. No. 4,252,885, comprising the sodium salt of5-sulfoisophthalic acid), copolymers having vinyl acetate units (see WO94/13481), polyvinyl acetates, polyurethanes, the alkali metal oralkaline earth metal salts of (C₁₀–C₁₈) fatty acids, copolymers havingunits comprised of butadiene and acrylonitrile, methyl methacrylate,methacrylic acid and/or acrylic acid and/or esters thereof. Thesubstances or compositions which give the additional functionality cancomprise the usual additives, such as antiblocking agents and/or pHstabilizers, their amounts preferably being from 0.05 to 5% by weight,with preference from 0.1 to 3% by weight.

The compositions or substances mentioned are applied in the form ofdilute, preferably aqueous solution, emulsion, or dispersion to one orboth sides of the film. The solvent is then removed. If the coatings areapplied in-line prior to the transverse stretching process, the heattreatment in the stretching frame is usually sufficient to volatilizethe solvent and to dry the coating. The layer thicknesses of the driedcoatings are then generally from 5 to 100 nm, preferably from 20 to 70nm, in particular from 30 to 50 nm.

In order to establish further desired properties, the film may also havebeen corona- or flame-treated. The manner of treatment is usually suchthat the surface tension of the film is then generally above 45 mN/m.

In another embodiment, the inventive film has been renderedflame-retardant. Flame-retardant means that in what is known as a fireprotection test the film complies with the conditions of DIN 4102 Part 2and in particular the conditions of DIN 4102 Part 1, and can beallocated to construction materials class B2 and in particular B1 forlow-flammability substances. The film, if appropriate renderedflame-retardant, is moreover intended to achieve at least the fireclassification VTM-2 in a UL 94 VTM fire test.

The film then comprises at least one flame retardant, which ispreferably fed by way of masterbatch technology directly during filmproduction, the proportion of this flame retardant generally being inthe range from 0.2 to 30% by weight, preferably from 0.5 to 25% byweight, particularly preferably from 1.0 to 20% by weight, based on theweight of the film. The proportion of the flame retardant in themasterbatch is generally from 5 to 60% by weight, preferably from 10 to50% by weight, based in each case on the total weight of themasterbatch. Examples of suitable flame retardants are organic brominecompounds, organic chlorine compounds, or organic nitrogen compounds, ormetal hydroxides or metal oxide trihydrates. However, a disadvantage ofthe halogen compounds is that in the event of a fire toxic and corrosivehydrogen halides can be produced. Another disadvantage is the low lightresistance of a film modified therewith.

It is significant that the flame retardant is soluble in the polymer orpolyester, because otherwise compliance with the optical propertiesrequired is not achieved.

Examples of other suitable flame retardants are organophosphoruscompounds, such as carboxyphosphinic acids, their anhydrides, anddimethyl methanephosphonate. Very suitable flame retardants here arethose in which the phosphorus compound has chemical bonding to thepolyester.

In one preferred embodiment, the inventive low-flammability filmcomprises not only polyester, preferably PET, the IR-reflective pigment,and the UV stabilizer, but also from 1 to 20% by weight of anorganophosphorus compound as flame retardant soluble in the polyester.Bis(2-hydroxyethyl)[(6-oxido-6H-dibenzo-[c,e][1,2]oxaphosphorin-6-yl)methyl]-butanedicarboxylatehaving the formula

is preferred as flame retardant.

Because the flame retardants generally have some degree of sensitivityto hydrolysis, additional use of a hydrolysis stabilizer can beadvisable. Examples of suitable hydrolysis stabilizers here arepolymeric carbodiimides. In this preferred embodiment, the inventivelow-flammability film comprises, as main constituent, crystallizablepolyester or PET, from 1 to 20% by weight of an organophosphoruscompound as flame retardant soluble in the polyester, and preferablyfrom 0.1 to 1.0% by weight of a hydrolysis stabilizer, based on theweight of the film.

The proportions described of flame retardant, and hydrolysis stabilizerhave also proven advantageous when the main constituent of the film isnot polyethylene terephthalate, but another polyester.

Measurements also showed that the inventive film does not become brittlewhen exposed to temperatures of 100° C. over a prolonged period.

The inventive film can moreover be thermoformed without predrying, andcan therefore be used to produce complex moldings.

The thermoforming process generally encompasses the steps of predrying,heating, molding, cooling, demolding, and heat-conditioning. Asurprising finding in the thermoforming process was that the inventivefilms can be thermoformed without predrying. This advantage overthermoformable polycarbonate films or thermoformable polymethacrylatefilms, for which the required predrying times, depending on thickness,are from 10 to 15 hours at temperatures of from 100 to 120° C.,dramatically reduces the costs for the forming process.

Examples of process parameters found for the thermoforming process were:

Step of process Inventive film Predrying not required Mold temperaturefrom 100 to 160° C. Heating time smaller than (<) 5 seconds per 10 μm ofthickness Film temperature from 160 to 200° C. during thermoformingPossible orientation from 1.5 to 2.0 factor Reproduction of detail goodShrinkage smaller than (<) 1.5%

Surprisingly, optical properties, particularly mattness, haze, andtransparency of the film, were found to remain homogeneous and unchangedduring thermoforming with an orientation factor of 2.0. This is probablyattributable to the fact that the lamellar, IR-reflective pigmentparticles have very substantial orientation parallel to the film surfacevia the high level of longitudinal and transverse stretching andsubsequent heat-setting in the film-production process.

The present invention also provides a process for producing theinventive film. The production process usually involves an extrusionprocess, for example on an extrusion line. It has proven particularlyadvantageous to add the IR-reflective pigment, the UV stabilizer, and,if appropriate, the other additives in the form of predried orprecrystallized masterbatches prior to the extrusion process.

In masterbatch technology it is preferable that the grain size and thebulk density of the masterbatches are similar to the grain size and thebulk density of the polyester used, thus achieving homogeneousdispersion, which gives homogeneous properties.

The inventive polyester films may be produced in the form of asingle-layer film (monofilm) by known processes from a polyester and, ifappropriate, from other polymers, and from at least one preferablylamellar, IR-reflective pigment and one UV stabilizer, and, ifappropriate, from other conventional additives (the latter in the usualamount of from 0.1 to 30% by weight, based on the weight of the film).One or both surfaces of the film may moreover be provided with afunctional coating by known processes.

Masterbatches which comprise the IR-reflective pigments shouldpreferably have been precrystallized or predried. The same applies tomasterbatches which comprise UV stabilizers, comprise flame retardants,and/or comprise other additives. This predrying involves gradual heatingof the masterbatches at reduced pressure (from 20 to 80 mbar, preferablyfrom 30 to 60 mbar, in particular from 40 to 50 mbar), stirring, and, ifappropriate, after-drying at a constant, elevated temperature (likewiseat reduced pressure). The masterbatches are preferably charged batchwiseat room temperature from a feed vessel in the desired blend togetherwith the polyester and, if appropriate, with other polymer componentsinto a vacuum dryer which during the course of the drying time orresidence time traverses a temperature profile of from about 10 to 160°C., preferably from 20 to 150° C., in particular from 30 to 130° C.During the residence time of about 6 hours, preferably 5 hours, inparticular 4 hours, the mixture of raw material is stirred at from about10 to 70 rpm, preferably from 15 to 65 rpm, in particular from 20 to 60rpm. The resultant precrystallized or predried raw material mixture isafter-dried in a downstream, likewise evacuated, container at from about90 to 180° C., preferably from 100 to 170° C., in particular from 110 to160° C., for from 2 to 8 hours, preferably from 3 to 7 hours, inparticular from 4 to 6 hours.

In the preferred extrusion process for production of the film, themolten polymer material is extruded with the other ingredients through aflat-film die, and quenched on a chill roll in the form of asubstantially amorphous prefilm. This film is then reheated and orientedlongitudinally and transversely, or transversely and longitudinally, orlongitudinally, transversely, and again longitudinally and/ortransversely. The stretching temperatures are generally above the glasstransition temperature T_(g) of the film by from 10 to 60° C., thelongitudinal stretching ratio is usually from 2.0 to 6.0, in particularfrom 3.0 to 4.5, the transverse stretching ratio being from 2.0 to 5.0,in particular from 3.0 to 4.5, and the ratio for any second longitudinaland transverse stretching carried out being from about 1.1 to 5.0. Thefirst longitudinal stretching may also be carried out simultaneouslywith the transverse stretching (simultaneous stretching). Heat-settingof the film follows at oven temperatures of from 180 to 260° C., inparticular from 220 to 250° C. The film is then cooled and wound.

It was surprising that in particular the use of the additives oringredients described and of masterbatch technology, combined with asuitable predrying and/or precrystallization process can produce anIR-reflective film with the required property profile without technicalproblems (such as caking in the dryer). During the production process,no, or almost no, deposits on the dies or condensation on the frames wasobserved, the result being that the inventive film has excellent opticalproperties, an excellent profile, and excellent layflat. It givesexcellent results when oriented and can therefore be producedcost-effectively in a reliable process.

It was surprising that the film has not only excellent thermoformabilitywithout any effect on optical properties but also exhibited a very smallchange, smaller than (<) 6, in yellowness index after 5000 hours ofWeather-Ometer weathering (WOM weathering), and that the IR-reflectiveeffect, the high light transmittance, and the scattering power forvisible light remain substantially unaffected. Even after this veryaggressive artificial weathering, transmittance for radiation in thewavelength range from 750 to 1300 nm (IR) is smaller than 50%.

Another very surprising fact is that the regrind can also be reusedduring film production without any adverse effect on the yellownessindex of the film. There is also no adverse change in the yellownessindex within the limits of measurement precision when comparison is madewith an untreated film.

The combination of their properties makes the inventive films suitablefor a wide variety of applications, for example in the indoor andoutdoor sector, in the construction sector and in the construction ofexhibition stands, in the fitting-out of shops and of stores, in theelectronics sector, and in the lighting sector, for greenhouses,exhibition requisites and promotional requisites, illuminatedadvertising profiles, protective glazing for machines and for vehicles,displays and placards, and for packaging, in particular ofheat-sensitive goods, with no restriction thereto.

Inventive examples are used below for further illustration of theinvention, which is not restricted thereto.

Test Methods

DIN = Deutsches Institut für Normung [German Institute forStandardization] ASTM = American Society for Testing and Materials ISO =International Organization for StandardizationLight Transmittance (Transparency)

Light transmittance is the ratio of total transmitted light to theamount of incident light.

Light transmittance is measured using ®HAZEGARD plus (Byk Gardener,Germany) test equipment to ASTM D1003.

Haze/Clarity

Haze is the percentage proportion of transmitted light that deviates bymore than 2.5° from the average direction of the incident light beam.Clarity is determined at an angle of less than 2.5°.

Haze and clarity are measured using ®HAZEGARD plus (Byk Gardener,Germany) test equipment to ASTM D1003.

Yellowness Index

The yellowness index of the film is determined to ASTM D1925-70 by meansof a Lambda 12 spectrophotometer (Perkin Elmer, US), D65 standardilluminant, 10° standard observer. Yellowness index is calculated fromthe measured standard color values X, Y, and Z by the following equationYID=[100·(1.28·X−1.06·Z)]/YSurface Defects

Surface defects were determined visually.

Mechanical Properties

Modulus of elasticity, ultimate tensile strength, tensile strain atbreak, and F₅ value are measured longitudinally and transversely to ISO527-1-2 with the aid of tensile test equipment (Zwick, 010, Ulm,Germany).

Standard Viscosity (SV) and Intrinsic Viscosity (IV):

Standard viscosity SV was measured—by a method based on DIN 53726—usinga 1% strength solution in dichloroacetic acid (DCA) at 25° C. SV(DCA)=(η_(rel)−1)×1000. Intrinsic viscosity (IV) is calculated asfollows from standard viscosity (SV)IV=[η]=6.907·10⁻⁴ SV(DCA)+0.063096[dl/g]Thermoformability

The films from inventive examples 1 to 5 and comparative examples 1 and2 can be thermoformed without predrying to give moldings on commerciallyavailable thermoforming machines, e.g. from Adolf Illig Maschinenbau(Heilbronn, Germany). Reproduction of detail in the moldings isexcellent, with a homogeneous surface.

Weathering (On Both Sides) and UV Resistance

UV resistance was tested as follows to the ISO 4892 test specification:

Test equipment Atlas Ci65 Weather-Ometer Test conditions to ISO 4892,i.e. artificial weathering Irradiation time 5000 hours (per side)Irradiation rate 0.5 W/m², 340 nm Temperature 63° C. Relative humidity50% Xenon lamp inner and outer filter comprised of borosilicateIrradiation cycles 102 minutes of UV light, then 18 minutes of UV lightwith water sprayed onto the specimens, then again 102 minutes of UVlight, etc.Fire Performance

Fire performance was determined to DIN 4102 Part 2, constructionmaterials class B2, and to DIN 4102 Part 1, construction materials classB1, and also in the UL 94 VTM test.

Gloss

Gloss was determined to DIN 67530. The reflectance was measured, thisbeing an optical value characteristic of the film surface. Based on thestandards ASTM D523 and ISO 2813, the angle of incidence was set at 20°and 60°. A beam of light hits the flat test surface at the set angle ofincidence and is reflected and/or scattered by this surface. Aproportional electrical variable is displayed representing light rayshitting the detector. The value measured is dimensionless and must bestated together with the angle of incidence.

Roughness

Roughness R_(a) of the film was determined to DIN 4768 with a cut-off of0.25 nm.

IR Permeability

The IR permeability of the film is determined on the basis of an IRtransmittance spectrum. An FTIR 1600 from Perkin Elmer, US was used. Thewavelength range measured extends from 750 to 1300 nm. The IRpermeability measured in % is the ratio of the amount of lighttransmitted to the amount of light incident in this wavelength range.

Diethylene Glycol Content, Polyethylene Glycol Content, and IsophthalicAcid Content

Diethylene glycol content, polyethylene glycol content, and isophthalicacid content is determined by gas chromatography after alkalinedigestion of the specimen.

Determination of Length of Vacuoles

The length of vacuoles in the film was determined on the basis ofscanning electron micrographs of microtom sections.

EXAMPLES

Each of the Examples and Comparative Examples below uses a single-layerfilm, produced on the extrusion line described.

The films were weathered to test specification ISO 4892 on both sides,in each case for 5000 hours, using the Atlas Ci 65 Weather-Ometer, andthen tested for discoloration.

The additives for achieving IR reflectance and for achieving theadditional functionalities were fed in the form of variousmasterbatches:

Masterbatch MB1 was comprised of 10% by weight of the lamellarIR-reflective pigment IRIODIN® SHR 870 (Merck, Germany) and 90% byweight of polyethylene terephthalate (RT49, KoSa, Germany). The bulkdensity of the masterbatch was 750 kg/m³.

Masterbatch MB2 was comprised of 20% by weight of2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol (TINUVIN® 1577 fromCiba Specialty Chemicals, Switzerland) and 80% by weight of polyethyleneterephthalate (RT49, KoSa, Germany). The bulk density of the masterbatchwas 750 kg/m³.

The following masterbatch was used to produce a flame-retardant film:Masterbatch MB3, a copolyester comprised of terephthalic acid, ethyleneglycol, and 30% by weight of bis(2-hydroxyethyl)[(6-oxido-6H-dibenzo[c,e][1,2]-oxaphosphorin-6-yl)methyl]butanedicarboxylateas further diol component (VA09, KoSa, Germany).

Example 1

A monofilm was produced and comprised polyethylene terephthalate as amain constituent, 20% by weight of MB1, and 5% by weight of MB2.

The standard viscosity SV (DCA) of the polyethylene terephthalate was810, corresponding to intrinsic viscosity IV (DCA) of 0.658 dl/g, itsdiethylene glycol content being 2.5%, and its isophthalic acid contentbeing 8.0%.

A mixture comprised of 75% by weight of polyethylene terephthalate, 20%by weight of masterbatch MB1 and 5% by weight of MB2 was charged at roomtemperature from separate feed vessels into a vacuum dryer whichtraversed a temperature profile of from 25 to 130° C. from the chargingtime to the end of the residence time. During the residence time ofabout 4 hours, the raw material mixture was stirred at 61 rpm.

The precrystallized or predried raw material mixture was after-dried inthe downstream hopper, likewise in vacuo, for 4 hours at 140° C. Theextrusion process described was then used to produce a single-layer filmof thickness 75 μm. The longitudinal and transverse stretching ratio setduring film production was λ_(L)=λ_(Q)=3.1. The stretching temperatureduring longitudinal and transverse orientation was T_(L)=T_(Q)=110° C.The temperature for the subsequent heat-setting was T_(S)=230° C.

Example 2

By analogy with Example 1, a film of thickness 75 μm was produced.Modifying Example 1, the film comprised 45% by weight of polyethyleneterephthalate, 20% by weight of MB1, and 5% by weight of MB2, and also30% by weight of recycled material directly associated with the process.

Example 3

By analogy with Example 1, a film of thickness 75 μm was produced.Modifying Example 1, the film comprised 55% by weight of polyethyleneterephthalate, 20% by weight of MB1, and 5% by weight of MB2, and also20% by weight of MB3.

Example 4

Example 1 was repeated. Modifying Example 1, the process parameters wereas follows: λ_(L)=λ_(T)=3.5; T_(L)=T_(T)=110° C.; T_(S)=230° C.

Example 5

Example 1 was repeated. Modifying Example 1, the process parameters wereas follows: λ_(L)=λ_(T)=3.1; T_(L)=T_(T)=95° C.; T_(S)=230° C.

Comparative Example 1 (CE 1)

Example 1 was repeated. Modifying Example 1, the film comprises no MB2.Proportion of PET correspondingly increased.

Comparative Example 2 (CE 2)

Example 1 was repeated. Modifying Example 1, the film comprises no MB1.Proportion of PET correspondingly increased.

Comparative Example 3 (CE 3)

Example 1 was repeated. Modifying Example 1, however, while the standardviscosity SV(DCA) of the polyethylene terephthalate used is 810 itsdiethylene glycol content is 0.6% and its isophthalic content is 0%.(RT49 from KoSa, Germany).

Comparative Example 4

Example 3 of WO 94/05727 was repeated.

The results obtained are shown in the table.

TABLE Property profile of films produced Properties E 1 E 2 E 3 E 4 E 5CE 1 CE 2 CE 3 CE 4 Appearance white white white white white whitetrans- white hazy matt matt matt matt matt matt parent matt Surfacedefect¹⁾ ++ ++ ++ ++ ++ ++ ++ ++ +− Transparency % 74 72 72 70 68 71 9072 85 Haze % 64 63 62 68 66 62 3 62 12 Clarity % 53 55 53 49 48 51 97 5489 Vacuoles from to μm 0.5–15 0.5–15 0.5–15 1.0–18 1.0–18 0.5–15 —³⁾0.5–15 —³⁾ 20° gloss − 9 11 10 7 6 10 —³⁾ 8 30 60° gloss − 28 29 27 2422 29 —³⁾ 29 81 Roughness R_(a) nm 321 318 325 398 378 335 9 343 182R_(t) nm 2369 2352 2379 2415 2402 2389 108 2398 1489 R_(z) nm 1807 17991812 1834 1827 1810 54 1822 1115 IR transmittance % 12 13 13 10 9 12 5610 57 Longitudinal N/mm² 4000 4100 3900 4300 4250 3900 4200 4300 2500modulus of elasticity Transverse N/mm² 5200 5100 4900 5300 5200 50005200 5250 4000 modulus of elasticity Longitudinal F5 N/mm² 100 105 95120 120 105 105 105 − Transverse F5 N/mm² 100 100 95 120 120 100 110 110− Longitudinal N/mm² 170 160 140 190 195 165 175 170 50 ultimate tensilestrength Transverse N/mm² 280 270 250 300 305 270 280 270 45 ultimatetensile strength Longitudinal % 170 165 145 140 145 160 180 165 10tensile strain at break Transverse % 90 95 80 80 85 90 100 105 5 tensilestrain at break Thermoform- − ++ ++ ++ ++ ++ ++ ++ − − ability¹⁾Yellowness index − 2.5 3.7 3.0 2.9 2.7 1.7 2.4 2.5 5.6 after productionYellowness index − 3.2 4.1 3.7 3.4 3.5 8.3 3.0 3.3 15.3 after 5000 h ofweathering UV absorption¹⁾ − ++ ++ ++ ++ ++ − ++ ++ − Fire − − − ++ − −− − − ++ performance¹⁾ ¹⁾− poor; +− acceptable; + good; ++ very good²⁾no vacuoles ³⁾not measurable

1. A biaxially oriented, thermoformable polyester film comprising a. acrystallizable polyester having a diethylene glycol content of greaterthan or equal to 1% by weight and/or an isophthalic acid content ofgreater than or equal to 2% by weight and/or a polyethylene glycolcontent greater than or equal to 1% by weight, b. at least oneIR-reflective pigment forming vacuoles and c. at least one UVstabilizer; wherein said film exhibits diffuse scattering behavior. 2.The polyester film as claimed in claim 1, wherein the crystallizablepolyester is polyethylene terephthalate, polyethylene naphthalate,polybutylene terephthalate or polytrimethylene terephthalate.
 3. Thepolyester film as claimed in claim 1, wherein the crystallizablepolyester has a diethylene glycol content of from about 1 to 10% byweight and/or an isophthalic acid content of from about 2 to 20% byweight and/or a polyethylene glycol content of from about 1 to 10% byweight.
 4. The polyester film as claimed in claim 1, wherein theproportion of crystallizable polyester is from about 50 to 99% by weightbased on the weight of the film.
 5. The polyester film as claimed inclaim 1, wherein the IR-reflective pigment is platelet-shaped.
 6. Thepolyester film as claimed in claim 1, wherein the IR-reflective pigmentis sheet silicate coated with metal oxides.
 7. The polyester film asclaimed in claim 1, wherein the film contains between 0.5 and 50% byweight of the IR-reflective pigment, based on the weight of the film. 8.The polyester film as claimed in claim 1, wherein the film has atransmission of less than about 50% for radiation in the wavelengthrange between 750 and 1300 nm.
 9. The polyester film as claimed in claim1, wherein the UV stabilizer is a 2-hydroxybenzotriazole, abenzoxazinone or a triazine.
 10. The polyester film as claimed in claim1, wherein the film comprises the UV stabilizer in an amount of fromabout 0.1 to 5% by weight based on the weight of the film.
 11. Thepolyester film as claimed in claim 1, which comprises a flame retardant.12. The polyester film as claimed in claim 1, which has one layer. 13.The polyester film as claimed in claim 1, wherein the film is coatedfunctionally on one or both surfaces.
 14. A process for preparing thepolyester film as claimed in claim 1, comprising the steps of a.producing a film by extrusion b. biaxially stretching the film and c.thermosetting the stretched film.
 15. A process for adjusting themattness of the polyester film as claimed in claim 1, which comprisesvarying the longitudinal and/or transverse stretching ratio, thestretching temperatures and/or the temperature of the thermosettingwithout changing the composition of the film.
 16. A thermally protectivecoating or packaging comprising a polyester film as claimed claim 1.